Hello,

We are currently working on fuzz testing feature, and we found a *heap-buffer-overflow* on `cflow`.

The stack traces are as follow:
```st
==2630==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x619000000990 at pc 0x5597a75897d7 bp 0x7ffe5e3a5b20 sp 0x7ffe5e3a5b10
WRITE of size 8 at 0x619000000990 thread T0
    #0 0x5597a75897d6 in collect_processor .../cflow-1.6/src/symbol.c:302
    #1 0x5597a759fa38 in hash_do_for_each .../cflow-1.6/gnu/hash.c:383
    #2 0x5597a7589af1 in collect_symbols .../cflow-1.6/src/symbol.c:334
    #3 0x5597a757fbde in tree_output .../cflow-1.6/src/output.c:383
    #4 0x5597a75802ce in output .../cflow-1.6/src/output.c:435
    #5 0x5597a757db93 in main .../cflow-1.6/src/main.c:829
```
The full stack trace is attached.

*Step to reproduce*

We configured `cflow` using `CFLAGS="-g -O0 -fsanitize=address" ./configure --disable-shared` and build it using `make -j10; make install`, and run it with:

```
./cflow <attached file> --number --include=x -n -o /dev/null
```

The input file is attached.

*Environment*
- OS: Ubuntu 18.04.5 LTS
- GCC version: gcc 7.5.0
- cflow version: [cflow 1.6](https://ftp.gnu.org/gnu/cflow/cflow-1.6.tar.xz)

Thank you.